The present invention relates in general to the production of synthetic films and more specifically to polypropylene films made by cold drawing a high melt strength, high xcex2-cystalline polypropylene. The resultant films are opaque, porous and have good physical properties.
Opaque polypropylene film can generally be obtained by stretching polypropylene containing a filler or colorant such as talc, clay, calcium carbonate or TiO2. It is known to those skilled in the art that xcex2-crystalline polypropylene can be used to produce opaque films without adding any filler or colorant. Such films owe their opacity to the presence of numerous pores throughout the film matrix that are formed by stretching the film.
Opaque films from xcex2-crystalline polypropylene can be prepared via xe2x80x9ccold drawingxe2x80x9d. That is, drawing the material at a relatively low temperature, such as more than 40xc2x0 C. below the melting temperature, e.g.,  less than 120xc2x0 C., as opposed to about 20 to 25xc2x0 C. below the melting temperature, e.g., 140xc2x0 C. for normal drawing, at a stretch ratio of 2-10 times in two directions. The first direction being the machine direction and the second direction being the transverse direction. However, it is difficult to produce a film with uniform thickness throughout because of the localized stress differences in the drawing directions, especially if thin gauge films ( less than 2 mil) are produced. As a result, the thickness of a film made from xcex2-crystalline polypropylene has been limited to  greater than 2 mil and the porosity limited to 30-40%. Another drawback to using a low stretching temperature is that it may lead to relatively high stretching forces that cause web breakage and machine limitation.
Therefore, a need exists in the art for a polypropylene capable of cold drawing to a thin gauge film which overcomes the above-mentioned drawbacks. The present invention provides a xcex2-polypropylene including a small amount of a very high molecular weight component. This xcex2-polypropylene exhibits high melt strength and is useful in making thin gauge films with high opacity and porosity at low drawing temperatures.
The present invention provides a composition of a xcex2-crystalline polypropylene with a K value of at least about 0.5 and a melt tension at 230xc2x0 C. of at least 5 cN, and 0.01 wt % to 10 wt % of a high molecular weight polymer having a molecular weight of at least about 1,000,000.
The present invention further provides a film made from a composition comprising a xcex2-crystalline polypropylene with a K value of at least about 0.5 and a melt tension at 230xc2x0 C. of at least 5 cN, and 0.01 wt % to 10 wt % of a high molecular weight polymer having a molecular weight of at least about 1,000,000.
The present invention yet further provides a method of making a porous film, the method comprising, incorporating 0.01 wt % to 10 wt % of a high molecular weight polymer having a molecular weight of at least about 1,000,000 into a polypropylene, the polypropylene having a K value of at least about 0.5 and a melt tension of at least about 5 cN at 230xc2x0 C., blending the polypropylene with a xcex2-nucleating agent, and drawing the polypropylene at a temperature about 20xc2x0 C. to about 50xc2x0 C. below the melt temperature of xcex2-crystals therein to a film having a thickness of about 0.1 to about 10 mil.
As used herein, the term xe2x80x9cpolypropylenexe2x80x9d generally includes homopolymers, copolymers of polypropylene, such as, for example, block, graft, impact, random and alternating copolymers, terpolymers, etc., and blends with other polymers, preferably polyolefins, such as high density polyethylene (HDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE), poly-1-butene, ethylene-vinyl alcohol copolymer or ethylene-methylmethacrylate copolymer and modifications thereof. Comonomers useful in the present invention include ethylene, 1-butene, 1-hexene, and other alpha-olefins.
Isotactic polypropylene is capable of crystallizing in several crystal forms. The xcex1, or monoclinic, form is the most prevalent one. The xcex2, or hexagonal, form is occasionally found in commercially available polypropylene, usually at low levels. The relative proportion, defined as the K value, of the xcex2 form in a polypropylene specimen can be determined by X-ray diffraction and is expressed by the empirical expression:   K  =            H      β                      H        β            +              H        110            +              H        040            +              H        130            
wherein H110, H040 and H130 are the heights of the three strong peaks (100), (040) (130) of the xcex1 form respectively and Hxcex2 is the height of the strong xcex2-peak (300). From the above expression, it will be apparent that in the absence of the xcex2-form the K value will be zero. Alternatively the K value will be one if only the xcex2-form is present.
The xcex2 crystalline content of the polypropylene used in the present invention should preferably be at least 50% (i.e., having a K value of 0.5 or more), more preferably at least 60% (K value of 0.6 or more) and most preferably at least 70% (K value of 0.7 or greater). Polypropylene with a high xcex2 content can be produced by any number of methods known in the art including, but not limited to, using the following xcex2-nucleators: Q-dye (the gamma-crystalline form of a quinacridone colorant, Permanent Red E3B), described in U.S. Pat. No. 5,310,584 issued to Jacoby et al., and the amides, such as N,Nxe2x80x2-dicyclohexane-2,6-naphthalene dicarboxamide, described in U.S. Pat. No. 5,491,188 assigned to the New Japan Company. In U.S. Pat. No. 5,231,126 Shi, et al describe xcex2-nucleating agents that are particularly preferred for use in the present invention. The xcex2-nucleating agents described by Shi et al are mixtures of organic dibasic acids with oxides, hydroxides or acid salts of Group II metals. Suitable dibasic acids include pimelic, suberic, azelaic, o-phthalic, iso-phthalic and terephthalic. Examples of suitable Group II metals are magnesium, calcium, strontium and barium.
xe2x80x9cDrawing ratioxe2x80x9d as used herein, also referred to as xe2x80x9cstretching ratioxe2x80x9d, means the ratio of the area of the film after drawing versus the area of the film, or sheet, before drawing and can be expressed as:       Drawing  Ratio    =            Area              [                  After          ⁢                      xe2x80x83                    ⁢          Drawing                ]                    Area              [                  Before          ⁢                      xe2x80x83                    ⁢          Drawing                ]            
This ratio can also be expressed in terms of the product of the ratio of the area in one drawing direction, such as the machine direction, before drawing, versus the area after drawing, times the ratio of the area in a second direction, such as the transverse direction, after drawing versus the area before drawing, and can be expressed as:       Drawing  Ratio    =                    (                  Area                      [                          Before              ⁢                              xe2x80x83                            ⁢              Drawing                        ]                          )                              (                      Area                          [                              After                ⁢                                  xe2x80x83                                ⁢                Drawing                            ]                                )                MD              xc3x97                  (                  Area                      [                          After              ⁢                              xe2x80x83                            ⁢              Drawing                        ]                          )                              (                      Area                          [                              After                ⁢                                  xe2x80x83                                ⁢                Drawing                            ]                                )                TD            
wherein MD refers to the machine direction and TD refers to the transverse direction.
Stretching ratios for the films of the present invention are preferably 2 to 50, more preferably 3-45 and most preferably 4-35. The films of the present invention have a porosity of preferably 10% to 75%, more preferably 15% to 50% and most preferably 25% to 45%. The films of the present invention can be stretched, or drawn down, to 0.1 to 10 mil, preferably 0.2 to 5 mil, more preferably to 0.5 to 2 mil and most preferably to about 1 mil.
High melt strength (HMS) polypropylene has been commercially available since at least the early 1980""s. xe2x80x9cMelt strengthxe2x80x9d as used herein means the resistance of elongational flow of a polymer melt and is characterized either by melt tension, i.e., the tensile stress of molten specimen or by elongational viscosity. Extensibility, which measures melt strength and drawability of a polymer melt can be determined by using a Rheoten extensional viscometer, in which a melt strand is extruded through a capillary die and pulled down with increasing velocity by a pair of wheels. The force necessary to pull the melt strand is measured to the breaking point of the strand. This maximum force is called melt strength or melt tension and the drawdown velocity at the breaking point is a measure of melt drawability.
High melt strength polypropylene is known to have good melt tension and drawdown capability in the melt state. xe2x80x9cHigh melt strength,xe2x80x9d as used herein is defined has a melt tension of at least 5 cN at 230xc2x0 C. Such a high melt strength polypropylene can be produced by introducing high molecular weight polymer chains, by broadening the molecular weight distribution, by branching or crosslinking via any means and by blending the polypropylene with other polymers. The drawdown capability of high melt strength polypropylene should preferably be present even in a semi-melt state of polypropylene, i.e., 20-50xc2x0 below the melting temperature of xcex2-crystalline polypropylene. Using the high melt strength polypropylene of the present invention comprising at least 50% xcex2-crystalline crystals, an opaque film  less than 2 mil having a desirable film properties such as uniform film thickness, opacity and porosity of 30-70% with good strength can be obtained without mechanical difficulty.
In the present invention, polypropylene can contain about 0.01 wt % up to about 10 wt % of high molecular weight polymers including, but not limited to, polyethylene and acrylic-modified polytetrafluoroethylene. These polymers preferably are present in the polypropylene in more than 0.01% and less than or equal to 10 wt %, more preferably in more than 0.05% and less than or equal to 5 wt % and most preferably in more than 0.1% and less than or equal to 2 wt %, to provide good drawing characteristics.
Polyethylene having a molecular weight above 1,000,000 to about 5,000,000, is termed high molecular weight. Polyethylene having a molecular weight above 5,000,000 to about 10,000,000, termed ultra high molecular weight. Techniques useful for dispersing the high and ultra high molecular weight polymer include melt blending (mechanically mixing or extruding the polymers while in a liquid state), and in-reactor blending. While both methods are operable with the current invention, the in-reactor blending method demonstrates superior dispersion efficiency. Preferably high and ultra high molecular weight polyethylene can be dispersed into the polypropylene matrix by polymerizing ethylene to a very high molecular weight before the propylene is polymerized. U.S. Pat. No. 4,271,279 details methods of making such high molecular weight polyethylene and its entire contents are incorporated herein by reference. In Example 1, the high molecular weight polyethylene was prepared in the first stage polymerization followed by the second stage polymerization of propylene. This is a typical example of in-reactor blending using multi-stage polymerization.
Acrylic-modified polytetrafluoroethylene, with a molecular weight above 1,000,000 or even above about 5,000,000, can be dispersed by mechanically blending with polypropylene in the same extruder which is used for subsequent film extrusion. A surface-modified acrylic-modified polytetrafluoroethylene particularly preferred in the present invention is commercially available as METABLEN(copyright) A 3000, from Japan""s Mitsubishi Rayon Company, Ltd.
The present invention will now be described for the purposes of illustration and not limitation by the following examples.
The results of Examples 1 through 3, and Comparative Example 4 are summarized in Table I.